Surface treatment for aluminum



Patented June 9,.1942

UNITED STATES- PATENT] OFFICE- SURFACE TREATMENT FOR.

Charles J..Slunder, Oakmont, 1a., assignor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application July 8, 1941,

Serial No. 401,465

4 Claims. (Cl. 148-43) treatment in the hydrofluoric acid-ferric chloride solution, the surface is removedand and aluminum base alloy surfaces usually result in the production of a frosted surface appearance that exhibits a definite metallic sheen or luster. This metallic appearance is undesirable for many applications where a minimum degree of'specular refiection'is desired. For example, where the' surface is to serve as a background for printing. In the past when such a flat white matte finish has been desired on aluminum or aluminum base alloy surfaces, it has usually been obtained by tjlliie use of pigmented paints, lacquers and the e. a c

It is an object of this invention to provide a simple and economical etching treatment for aluminum and aluminum base alloy surfaces which will provide a surface having a white matte finish free from apparent metallic luster when viewed from any angle.

It is a further object of the invention to provide an etching treatment to produce a white matte finish on such surfaces which is permanent in nature and will not smudge when subjected to handling,

A still further object is the provision of a method of surface treatment that will produce a flat white appearance on aluminum and aluminum base alloy objects which makes it possible to obtain a uniform white appearance over the entire surface regardless of the extent of the area of the surface. Other further objects will present themselves upon consideration of the specification and claims when taken in conjunction with the spe-' cific examples of the method of the invention given herein.

aluminum base alloy article or surface which,

' ment.

cleaned and subsequently again immersed for a period in the hydrofluoric acid-ferric chlor de etching solution. The surface is again removed from the solution and treated in an acid clean- I ing solution and finally washed in water.

In the practice of the invention the aluminum or aluminum base alloy surface to be treated is immersed in the etching solution, which is pref erably employed at a temperature of between 80 F. and 100 F. While the etching solution is preferably used at or near this temperature, it may be employed at higher or lower temperatures with suitable modifications of the treat- The concentration of the hydrofluoric acid-ferric chloride solution to obtain the best results is rather critical. In general; the best results are obtained witha solution containing when necessary, has been cleaned to insure the removal of any oil, grease or other foreign material in any suitable manner is subjected to an etching treatment in a solution containing hydrofluoric acid and ferric chloride. After the from 10 to 15 per cent by weight of-hydrofiuoric acid (HF) and 5 to 15 per cent by weight of ferric chloride (FeCla-BHzO). Upon its initial immersion in the etching solution, the aluminum or aluminum base alloy surface will be vigorously etching solution, and it has been observed that the reaction during this second immersion is less violent than the reaction during the initial immersion. After a period of about /2 to 1 minute, the reaction appears to-come to a complete stop. Upon removal from the etching solution the surface will be found to be uniformly covered with a light gray layer which appears tube a definite coating on the aluminum or aluminum base alloy surface.

This light graylayer formed in the second etching step may be removed by a number of acid solutions which are customarily used for the removal of coatings or discoloration from aluminum surfaces. Among the best of the acid solutions used for this purpose are concentrated nitric acid solutions, solutions containing both nitric and sulfuric acids and hot chromic and sulfuric acid solutions. Both chromic and nitric acids appear to have inhibiting effects on aluminum and aluminum base alloy surface and they are usually found as constituents of such cleaning or acid stripping solutions.

per liter sulfuric acid (H2804).

Suitable stripping solutions may, for example, consist of nitric acid having a concentration of 750 grams HNOa per liter. This solution may be used cold. An alternative solution may contain 500 grams per liter of nitric acid (HNOa) and 1'70 grams This solution may also be used cold. Another solution frequently employed contains 175 grams per liter of sulfuric acid (H2804) and 35 grams per liter chromic acid anhydride (CrOa). This latter solution is preferably employed at a temperature of 170 F. to 180 F. to give the best results.

After the second etching step, the aluminum or aluminum base alloy surfacebeing treated is immersed in one of the above mentioned cleaning or stripping solutions under the described conditions, and after removal from this stripping or cleaning solution it may be again washed in water. The light gray layer remaining on the be completely removed in the. cleaning or stripping solution and the surface will be found to be practically .dead white with no visible metallic luster when viewed from any angle. The surface will be found to be smooth and uniform.

- While the above described treatment can be employed advantageously on any aluminum or aluminum base alloy surface, the best results from the standpoint of producing a decorative white matte surface will be obtained when the aluminum or aluminum base alloy, among its material was then removed and again washed in water; The surface was found to have a practically dead white matte finish with no metallic luster apparent when viewed from any angle.

It will be understood, of course, that while this invention has been described with reference to certain preferred embodiments thereof, various modifications and changes may be made therein within the spirit of the invention as set forth in the appended claims.

ish on aluminum and aluminum base alloys which consists ui immersing the surface in a solution containing 10 to 15 per cent by weight face again in said solution and again removing ,surface afterthe second etching treatment will of hydrofluoric acid and 5 to 15 percent by weight of ferric chloride, removing said surface and cleaning the same of the products of the reaction with said solution, immersing the sursaid surface from the solution and subsequently cleaning and stripping the surface of the discoloring products of the reaction in an acid solution.

2. The method of producing a white matte finish, on aluminum and aluminum base alloys which consists in immersing the surface in a .solution containing 10 to 15 per cent by weight of hydrofluoric acid and 5 to 15 per cent by weight of ferric chloride at a temperature of from 80 to about 0.16 .per cent copper was immersed in a tion, 150 grams of'ferric chloride crystals and enough water added to make 1000 cc. of total solution. The temperature of the solution was held at about 90 F. to 100 F. The sheet was etched in this solution until the reaction had practically stopped. This treatment in the etching solution took a total time of about 3 minutes. The material was then removed from the etching solution, washed thoroughly with water, and returned to the etching solution and allowed to etch again for about y minute, after which time the reaction of the solution with the metal had practically stopped. The material was then removed from. the solution and again washed in water. The gray layer or coating on thema- 100" Fahrenheit, removing said surface and cleaning the same of the products of the reaction with said solution, immersing'the surface again in said solution and again removing said surface from the solution and subsequently invention, a piece of aluminum sheet containing cleaning and stripping the surface of the discoloring products of the reaction in an acid solution.

consists in immersing the surface in a solution containing 10 to 15 per cent by weight of hydrosurface from the solution and subsequently.

5 cleaning and stripping the surface of the 'discoloring products of the reaction in a cleaning solution containing a material selected from the group consisting of chromic andnitric acids.

4. The method of producing a white matte surface on aluminum and aluminum base alloys which consists in immersing the surface in a terial was then removed by immersion in a solution of nitric acid which had a concentration solution containing 10 to 15 per cent by weight of hydrofluoric acid and 5 to 15 percent by weight of ferric chloride, removing said surface surface again in said solution and again removing said surface from the solution and subsequently cleaning and stripping the surface of the discoloring products of the reaction in an acid cmams J. sLUNnEa.

a. The method of producing a white matte fin- 40 ish on aluminum and aluminum base alloys which and washing the same in water, immersing the 

